Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
Organic optoelectronic devices such as thin film transistors (TFTs), light emitting diodes (LEDs) and photovoltaic (PV) cells, have gained considerable attention of researchers during the past decade. Organic semiconductors can be deposited on a variety of substrates, which potentially simplifies and lowers fabrication costs when compared to inorganic semiconductors. However, the unique processing requirements of organic semiconductors can also limit their application. For example, light emitting devices and PV cells typically consist of thin (<100 nm) films of either conjugated polymers or monomers, sandwiched between conducting electrodes. For full-color displays and multi-transistor circuits, the active organic layers themselves must also be laterally patterned. However, the organic layers are typically too fragile to withstand conventional semiconductor processing methods such as photolithography, plasma processing, or reactive ion etching. Many fabrication and patterning techniques have therefore been developed to address these unique requirements, emphasizing primarily the ease and low cost of processing.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be an fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
Early methods of patterning organic materials involved the deposition of organic materials through a mask. The organic materials may be deposited through an “integrated” mask which is attached to the substrate on which the device is being fabricated, as disclosed in U.S. Pat. No. 6,596,443, issued on Jul. 22, 2003, which is incorporated by reference in its entirety. Or, the organic materials may be deposited through a shadow mask that is not integrally connected to the substrate, as disclosed in U.S. Pat. No. 6,214,631, issued on Apr. 10, 2001, which is incorporated by reference in its entirety. However, the resolution that may be achieved with such masks is limited due to a number of factors, including the resolution to which a mask may be reliably fabricated, the buildup of organic material on the mask, and the diffusion of organic material in between the mask and the substrate over which it is being deposited.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.